Modulation of high-frequency energy



Jan. 1, 1952 J, 1 STICKLER 2,580,970

MODULATION OF HIGH-FREQUENCY ENERGY Filed sept. 1o, 1949 INVNTORATTORNEY Patented Jan. 1, 19,52

MODULATION OF HIGH-"FREQUENCY `ENERGY ohn J. Stickler, Ithaca, N. Y.,assignorwto Radio Corporation of America, a corporation ofDelawareAppiica'tionseptembr 1o, 1949, sentano. 115,051

This. invention relates to the modulation of ultra high kfrequencyelectromagnetic energy.

One of the more serious problems encountered in the modulation of ultrahigh frequencylenergy is the variation in impedance which is presentedto the generator of the high frequency energy. This variation ofimpedance causes various clifculties, for example, if the high frequencygenerator is a magnetron the modulation becomes seriously non-linear. Ingeneral, it is desirable in themodulation of energy from ultra highfrequency generators to maintain constant the impedance into which thegenerator works, thereby reducing standing wave losses and avoidingother diiiiculties.

It i s an object of the present invention to provide a modulator of highfrequency energy which aords a substantially constant output impedancefor the generator. f

It is another object of the invention to provide a novel means ofmodulating ultra high frequency energy.

A further object of the invention is to provide a novel ultra .highfrequency modulator Vof the absorption type.

These and other objects, advantages, and novel features of the inventionwill be more apparent from the following description when taken inconnection with the accompanying drawing in which like referencenumerals refer to like parts and 'in which 'Figure 1 is a partiallyschematically' circuit diagram of one embodiment of the invention inwhich a single cavity resonator is employed;

Figure lais a cross-sectional View of another cavity resonator which maybe employed in place of the resonator of Figure 1; ,Y

Figure 2 is a cavity resonator system employing two cavity resonatorswhich may be used in the system of Figure l; and

Figure 3 is a partially schematically circuit diagram of anotherembodiment of the invention in which two cavity resonators are coupledby an electron beam. A f

In accordance with the invention', I provideia system in which theenergy from the generator is coupled through a particularly desirablenetwork arrangement, to a cavity resonator system, the energy preferablyexciting the resonator system in two independent modes both in time andphase quadrature. An electron beam absorbs energy' from the resonator.In a preferred embodiment the remainder of the energy which is notabsorbed by the beam is transmitted, the ,modulation being secured .byVariation of the electron 11 claims. (ci. 25o-"17) v beam absorption. Inanotherembodimentof the invention the energy absorbed by the `beam isreleased to excite a second cavity resonator from which'the energy isabstracted'to be transmitted.

Again, the modulation is secured by variation of the electron beamabsorption. l Referring now more particularly to Figure 1,

` the energy from generator I is coupled to a coaxial transmission lineI2. Transmission line I2 is connected to a diplexer network I .t whichmay be the same as the network disclosed rand illustrated inthe patentto G. H. .Brown (2,454,907) issuedNovember 30,k 1948, and entitled RadioFrequency Network. A two-wire diplexer network is disclosed rin the RCAReview for June 1949, volume X, No. 2 in the article by O. M. Woodward,Jr. entitled Reversible-beam antenna .for twelve-channel televisionreception. The network comprises in addition to the line I2 leading intothe network, an outer sleeve IIS, a second coaxial transmission line I8the outer conductor of which is connected to the sleeve I6 and the'innerconductor'of rwhich is connected to the outer `vthe terminationsof the slotted portions of the outerconductor of line I2. vDiplexers areknown for hollow pipe waveguide transmission lines if these should bepreferred.

A cavity resonator 36 is provided, which, in the present example, isrectangular having plana-r metallic walls which are normal to mutuallyperpendicular axes X, Y, and Z and therefore having also edges parallelto ther said axis. In this cavity resonator 3e, a cross section normalto the Y axis and parallel' to the Xv-Z plane, is square. The' lines 26and 28 are' coupled respectively through adjacent sides 32' and 34 whichare nor'- mal respectively tothe X and Z axes atcor'respondingmid-points of the side walls 32, 34.

3 due to variations from the geometrical symmetry of construction andthat due to the resistance effect of the walls is well known.Independent coupling to two such modes in a cavity resonator isdescribed, for example, in the patent to Philip S. Carter (2,337,184)issued December 2l, 1943, and entitled Coupling Circuit. One of thelines 26, 28 is a quarter wavelength longer than the other line asmeasured from the end of the slotted portion 22 of the extension oftransmission line I2. More generally, the terminal couplings tocorresponding points 36, 38 in the resonator are in phase quadraturewith respect to each other. Means are provided to produce an electronbeam which is initially directed along a central line through the cavityresonator 30 in the direction of the Y axis. Such a means may comprisean electron gun structure 40 including a cathode 42' and a control grid44 for varying the intensity of the beam. Various refinements are knownfor producing a Well collimated beam of electrons by 'such means andtherefore the means are only schematically disclosed. lIhese means mayfurther include an envelope 46 which extends entirely through apertures49 and 5I the respective sides 50 and 52 Vwhich are normal to the Yaxis. I prefer to couple the points of coupling 3s and 38 so that theoutput impedance of the lines 26, 28 is substantially identical and thisis easily vaccomplished by coupling at the center point of each side.The cavity resonator 30 may be square, or its length in the direction ofthe Y'axis may be less than that on the X and Z axes. Itis preferred toexcite by the couplings 36 and 38 the dominant modes which have electricvectors respectively in the Z and X directions. An anode 53 is providedwithin the envelope 46 to collect the beam of electrons and suitablevoltage supplies are provided of which only the B-iand B- are indicated,it being understood that a bias supply may be required for the controlgrid 44 or other grids. The control grid 44 may be connected to a source|38 of modulating signal, as schematically indicated, to control theelectron ow.

Turning now to the operation of the device, energy supplied by thegenerator I passes through the transmission line I2 to the diplexernetwork I4. All of such energy passes through the lines 26 and 28, andwith proper design of the diplexer is divided equally therebetween inphase opposition. This is because the quarter wave slotted portion 22effectively presents a high impedance to current flow down the sleevedportion, and therefore the two points at which the inner conductors oflines 26 and 28 are attached 4are at a high radio frequency potentialwith respect to each other. Upon reaching coupling points 36 and 38 theenergy excites two dominant modes of resonance of the same frequency.Both of these modes have electric vectors normal to the direction oftravel of the electron beam, and

in both phase and space quadrature, resulting in circular polarization.These relations are preferable because equal absorption by the beam fromeach field results. A magnetic eld (the'means for providing which areindicated as poles of a magnet marked N, S respectively) is orientedwith the magnetic lines of force parallel to the Y initial direction oftravel of the electrons from the gun structure 40, that is, parallel tothe Y axis. The electron beam interacts with the elds in thecayityresonator to absorb energy therefrom, each electron being thereby givena sp'iral path terminating on anode 53 with an increased total velocity.It is apparent that a portion of the energy, however, will not beabsorbed by the electron beam. The greater the number of electrous, thegreater the absorption and if no electrons, there will be no absorption.The energy which is not absorbed by the beam by the respective modeswill be reflected from the points of coupling. The changes in resonatorimpedance as presented to lines 26, 28 respectively are equal, as thebeam absorbs equally from each mode, as mentioned above. Since the modesare not coupled, substantially all of the energy excited and unabsorbedby the beam, for example, at coupling point 36, Vwill be reflected intoline 26 at Vpoint 36. At point 38, similarly, the unabsorbed energy fromthe mode there excited will be reiiected into line 28. The phases of thereflected energies, because of symmetry considerations, and because ofthe phase quadrature relations at points 36, 38 will necessarily be inphase quadrature. This energy now returns down the lines 26, 28 andarrives at the diplexer I4 in phase. It will be obvious to those skilledin the art that at the central operating frequency, such energy will nowtravel out the transmission line I8 to the antenna 66 which is hereschematically represented as a simple type of dipole radiator. Thisbecause the slotted portion at the end of which the center conductor ofline I2 connects is at the same radio frequency voltage for in-phaseenergy Aas the other side of the slot. Therefore, no difference involtages in the two members of the line I2 is caused by Such energy.Accordingly, substantially none of the energy is returned to thegenerator I0. The operation described is equivalent to saying that, ifthe diplexer. I4 is properly matched to the transmission line I2, andthe transmission line I2 is properly matched to the generator IU therewill be no Lreflections or standing waves on the transmission line I2and that the generator Il) will be working into a constant and perfectlymatched impedance, regardless of the amount of energy absorption. Theenergy absorbed by the electron beam is, of course, dissipated as heaton the anode 53.

It will be clear that other types of cavity resonators could be employedwhich have modes of resonance in which the electric vectors are normalto each other and in which the two inputs from lines 26, 28 wouldterminate in equal impedances. For example, referring'to Figure la, acavity resonator 30' having a circular cross section in which the pointsof coupling are displaced from the axis at points A and B may beemployed. In this case, the dominant modes are preferably excited, thetwo modes respectively having electric vectors as illustrated by thebroken and full lines. Such modes are independent and have the samefrequency of resonance.

With respect to the magnetic field oriented along the Y axis, the fieldstrength thereof should correspond to the cyclotron frequency. Thecyclotron frequency for a magnetic field strength H is defined herein asbeing related to H by the formula:

f wifi-fifi Y where lil m equal to a period of oscillation of ltheelectric eld. There :are certain very obvious modifications which mightbe desirable in the device of Figure 1.. For example, the side 52 neednot be apertured Vbut may itself serve as the'anode, as shown in Figure12 hereinafter. VInstead of the envelope 46 extending through thisaperture and through the cavity resonator, the cavity resonator itselfmay be evacuated and the transmission lines 26, 2-8 dieiectricallyvacuum sealed respectively inknown manner at their points of entry orcoupling 35, 381to the Vside walls :32, 34. The gun structure maybeenclosed outside the cavity and its envelope sealed to the aperture inthe wall 50, as the Lguns 4i3!! and |36 in Figure 2te their respectivecavity resonators. The cavity resonator is then maintained at B-l-vpotential so that the electrons are not accelerated during their passagethrough the cavity resonator except by their interaction witht he eldstherein. In connection with the proper strengthof the magnetic field anda further explanation of absorption of energy by an electron beam,reference may be made to an article by C. L. Cuccia entitled Theelectron coupler-a developmental tube for amplitude modulation and powercontrol at ultra-high frequencies in the RCA Review, volume X, No. 2 forJune 1949, beginning at page `270. e

If phase modulation is desired, it may'be secured by using a differentvalue of magnetic iield strength than theA cyclotron value. As wellknown, there is a second value of eld strength which makes the cavityresonator appear purely reactive to the coupled lines rather than purelyresistive. If this second value of magnetic field strength is used, theoutput is phase modulated with electron beam intensity variations. Witheither value of magnetic field strength, the change in beam intensitycauses a change in impedance of the cavity resonator resulting inmodulation. This second value of field strength is well known in theart, although it does not beal1 an appellation such as is borne by thecyclotron value as interrelated with the frequency, in accordance withthe equation above, so far as applicant knows.

Referring now to Figure 2, 'it will be apparent that two separatecavities |30 and |32 might be utilized instead of the single cavity, andwill function as equivalents thereof, provided the electron beams andmagnetic eld values would be the same in each case and the beamsmodulated in the same fashion. Such an arrangement is illustrated inFigure 2 in schematically perspective view, with the transmission lines2B, 28 leading respectively to the two cavities which have the sameresonant frequency for the particular mode coupled at the termination ofthe lines 28, 218. Again, lines 26 and 28 differ by a quarter wavelengthat the operating frequency and lead to a network as shown in Figure 1.Electron guns |34, |36 are arranged to produce a beam of electrons ineach cavity together with means for providing a magnetic field parallelto the initial direction of the electron beam produced by the guns. Thegrids of the electron guns |34, |36 may be tied together to receive amodulating signal from a common source |38.

Figure 3 shows a quite diierent modication of the invention comprising agenerator l0, a transmission line |2 coupled thereto as before, anetwork I4 which may be the same as the network I4 of Figure 1, a pairof transmission lines 26, 28 coupled to the network as before,

and a transmission ,line |8' coupled` to the net-` workV :iii as Ybeforeand Aterminated f in a resistor Rl. Two cavity resonators 230 and 232are provided having a Ycommon wall 234 in which is a communicatingaperture236 of small diameter. An electron beam illustratedby a dottedline 238 couples the two cavities by being injected at the wall 2400irectangular cavity resonator 230 land. being directed to pass throughthe aperture :236 toward the far wall 242 of rectangular cavityresonator 232, the wall 242 serving asan anode for the beam. The beammay be modulated in intensity by a suitable ygrid (not shown :in thisfigure). As'before, the lines 26 yand 28 are coupled at points 2M and245.1'espectively by' cavity resonator 230 to excite `modes which havethe same frequency .of resonance and -which also have electric vectorsnormal to the axisr ofthe electron beam 23,8. VCavity resonators v23|)and 232 are similar and it will be apparent `thatthe energy absorbedbythe electron vbeam l238v is coupled to two similar modes in cavityrresonator 232. Thus'the energy absorbed in cavity resonator 23S iscoupled to cavity resonator 232 to excite two similar. modes therein.VThe energy is picked up by two transmission lines 248 and 25S at points252 and 251i respectivelyand which are coupled as before to twoindependent modes having the same frequency of resonance as the modes inresonator 230 and the same frequency of resonance as each other.Transmission lines Eris and 2t@ are coupled to the diplexing network2.53 which is similar to the network I4 and coupled to two furthertransmission lines 258 and 2te, the latter of which leads toan antenna2%2 and the other of which is terminated in a matched absorbing resistorR2.

In operation, the generator i0 works into the transmission line i2 tocou-ple energy in phase opposition to the transmission lines 26 and 28.Due to the quarter wavelength difference in the lengths of lines 26 and23, the energy is coupled into the cavity resonator 23% and absorbed bythe electron beam 228 which in turn couples the energyto the modescavity resonator 23-3 -where it is picked up by lines 24B and 256.Depending on which of the iines 24S `and 25|) is longer, the energyarrives at diplexer- 2% either in phase or in phase opposition. Assumingin this case that itarrives in phase, it will be transmitted throughtransmission line 2iiii to antenna 262. Any out of phase components,that is, componentsarriving in phase opposition will be absorbed by theabsorbing resistor R2. As before, generator it works into affixedimpedance. It will alsoY be apparent that transmission line 268 andantenna 292 see a constant impedance.

It will be obvious to those skilled in the art that in Figure 1generator it may be connected to transmission line is and antenna titvto transmission line |21, and the operation of the device in that eventwill also be obvious to those skilled Vin the art from what has beensaid heretofore.

What I claim is:

l. A circuit comprising rst and second pairs of transmission lines, anetwork therebetween coupling at the center operating frequency thefirst of said rst pair of lines to said second pair of lines in, phaseopposition at the points of connection of said second pair to saidnetwork and coupling the second of said first pair of lines to saidsecond pair of lines in phase at said points,`

a resonator system having two independent resonant modes of oscillationat said frequency, each of said second pair of lines being coupled tosaid resonator system means coupled to said resonator system .to effectrespectively said modes to change equally the impedance as presented toeach of said second pair of lines by said resonator system, each line ofsaid second pair being coupled' to affect respectively one and not toaffect the other of said modes and one of said second pair of linesbeing a quarter wavelength longer from its said point of networkconnection to its effective point of coupling to said resonator systemthan the other, whereby energy applied to the circuit through one lineof said first pair isrtransmitted to said second line of said first pairin an amount depending on the changes in said impedancespresented tosaid lines.

2. A circuit comprising first and second pairs off-transmission lines, anetwork therebetween coupling at the center operating frequency thefirst of said rst pair of lines to said second pair of lines in phaseopposition at the points of connection of said second pair to saidnetwork and coupling the second of said iirst pair of lines to saidsecond pair of lines in phase at said points, a resonator systemcomprising a cavity resonator having two independent resonant modes ofoscillation at said frequency the electric vectors of which are 'normalto each other, means coupled to said vresonator system to aifectrespectively said modes to absorb equally energy therefrom andcomprising an electron beam having its axis normal to said vectors and amagnetic field, said magnetic iield being substantially uniformthroughout the electron travel through the cavity resonator, each lineof said second pair being coupled to said resonator system to aiectrespectively one and not to affect the other of said modes and one ofsaid second pair of lines being a quarter wavelength longer from itssaid point of `network connection to its effective point of coupling tosaid resonator system than the other, whereby energy applied to thecircuit through said first line of said first pair is transmitted tosaid second line of said first pair in an amount depending on theimpedance presented to said lines by said resonator system.

3. The circuit claimed in claim 2, the strength of said magnetic fieldbeing substantially equal to the cyclotron value whereby said impedanceis substantially purely resistive.

4. lThe circuit claimed in claim 2, the strength of said magnetic fieldbeing substantially equal to the value required to make said impedancesubstantially purely reactive.

5. The circuit claimed in claim 2, Vfurther comprising a generatorcoupled to one of said first pair of lines, and an antenna coupled tothe other of said rst pair of lines.

6. A circuit comprising first and second pairs of transmission lines, anetwork therebetween coupling at the center operating frequency thefirst of said first pair of lines to said second pair of lines in phaseopposition at the points of connection of said second pair to saidnetwork and coupling the second of said rst pair of lines to said secondpair of lines in phase at said points. a resonator system comprising arst cavity resonator having two independent resonant modes ofoscillation at said frequency and a second cavity resonator similar tosaid first and having two similar independent resonant modes ofoscillation at said frequency, energy coupling means comprising anelectron beam directed through said first resonator and thence throughsaid second resonator and a means to provide a substantially uniformmagnetic field axially of said beam and having the cyclotron value forsaid frequency throughout the travel of said beam through saidresonators, said beam being coupled to said first cavity modes to absorbequally energy therefrom and being coupled to said second cavity modesto impart energy thereto, each line of said second pair of lines beingcoupled to one and not to the other of said first cavity resonator modesand one of said second pair of lines being a quarter wavelength longerfrom its said point of network connection to its effective point ofcoupling to its respective mode than the other, third vand fourth pairsof transmission lines, a second network therebetween coupling at thecenter operating frequency the first of said third pair ,of lines tosaid fourth pair of lines in phase opposition at the points ofconnection of said fourth pair to said second network and coupling thesecond of said third pair of lines to said fourth pair of lines in phaseat said points of connection of said fourth pair, each line of saidfourth pair being coupled to one and not to the other of said modes ofsaid second resonator and one of said fourth pair of lines being aquarter wavelength longer from its said point of second networkconnection to its effective point of coupling to its respective modethan the other, whereby energy applied to the circuit through one lineof said first pair is transmitted to one line of said fourth pair in anamount depending on the absorption thereof by said beam in said firstcavity resonator.

7. The circuit claimed in claim 6, further comprising a generatorcoupled to one of said first pair of lines.

8. The circuit claimed in claim 2, said cavity resonator being arectangular cavity resonator having a square cross section.

9. The circuit claimed in claim 2, the said resonator being a circularcavity resonator.

10. The circuit claimed in claim 1, said lines being co-axialtransmission lines.

l1. The circuit claimed in claim 1, said resonator system comprising twoseparate and similar cavity resonators.

l JOHN J. STICKLER.

REFERENCES CITED The following references are of record in the vle ofthis patent:

UNITED STATES PATENTS Australia Jan. 21, 1943

